Hydrocarbon bearing diatomaceous formations are unique because they have high porosity, but have little permeability, i.e., they may contain significant amounts of oil but have very small flow channels or limited fractures through which oil could flow and, ultimately, be recovered. The unique properties of this type of formation arise because of its composition. The formation is composed largely of skeletal remains of diatoms. These skeletons are small. Also, their fragments are hollow and fluids may flow through natural micropores. They may also flow through fractures and openings between the diatom frustules.
Oil recovery from diatomaceous formations is usually quite limited because a significant portion of oil saturation may be bypassed using conventional recovery techniques such as primary, waterflooding, cyclic steam, or steam drive. Significant improvement of oil recovery would require that a method of displacing oil from the interior of the diatoms into the flow channels between the diatoms be provided. Furthermore, an additional benefit would be derived from improving permeability in the natural flow channels so that the oil can be recovered more rapidly and with a higher sweep efficiency.
Various methods have been proposed for increasing crude oil production from diatomaceous formations. For Example, U.S. Pat. No. 4,485,871 teaches a method of recovering hydrocarbons from diatomite in which an alcohol is injected into the formation followed by an aqueous alkaline solution. However, it has been found that certain formations do not respond to this type of stimulation. In particular, oil recovery using this method is not optimum in formations which are deeply buried and have not been extensively exposed to the atmosphere or oxygen bearing formation water. The interfacial tension and oil/rock wettability in these deeply buried formations must be modified in order to enhance oil recovery. A related method is described in U.S. Pat. No. 4,374,023.
U.S. Pat. No. 4,167,470 teaches an alternative method of recovering oil from diatomaceous formations in which a hydrocarbon solvent is contacted with mined diatomaceous ore in a 6 stage extraction process. Solvent is then recovered in a steam stripping apparatus. There are several problems in utilizing this process in a cost effective manner. For example, the ore must be mined, with the attendant environmental and economic considerations. Furthermore, the process is extremely complex and capital intensive. The process is an extraction process and discloses no provision for adding, for example, a chelating agent, and it is not a two-phase process such as is disclosed herein.
British Pat. No. 1,559,948 (Buckley) teaches a method of recovering heavy oil from tar sand (which is sometimes incorrectly attributed to being equivalent to diatomite), in which a steam distillable solvent and an oil soluble surfactant are simultaneously injected. The solvent is variously described as being, for example, kerosene or naphtha. The surfactants are described as being in one of two classes, i.e., thermally labile or steam distillable. An oil soluble surfactant such as thermally labile surfactants are suggested as being alkylaryl sulphonates or alkaryl naphthenic sulphonates. Examples of such steam distillable surfactants are suggested as being alkyl and other amines. Again, this is an extraction process and does not suggest a more preferred two-phase process.
Davis in U.S. Pat. No. 4,828,031 that issued on May 9, 1989 discloses a method for recovering oil from a diatomaceous formation. A solvent is injected into the diatomaceous formation followed by an aqueous surface active solution. The aqueous surface active solution contains a diatomite/oil water wettability improving agent and an oil/water surface tension lowering agent. The method may be supplemented by injection of water and/or steam into the diatomite. Injection pressure into the formation was held to just below the value where a long fracture may be induced.
Therefore, what is needed is a method that enhances an imbibition process to increase the rate of water uptake by a diatomaceous reservoir where a series of wells are utilized along with hydraulic fracturing.